This application pertains to the art of radiant energy detection and more particularly, to apparatus for converting variations in incident, gamma or x-radiation into corresponding variations in an electrical property, such as changes in output voltage, current or resistance. The invention is particularly applicable to computerized axial tomographic scanners and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications such as industrial flaw detectors and other apparatus which detect radiation with high resolution.
Generally, a computerized axial tomographic scanner comprises a source of radiation for irradiating a patient and a plurality of radiation detectors positioned opposite the patient from the radiation source. The detectors receive radiation beams which have passed through the patient along known paths. At least the radiation source is movably mounted for irradiating the patient from a plurality of directions. The detectors are positioned to detect the radiation beams along a plurality of interesecting paths through a planar slice of the patient. With well-known computer reconstruction techniques, the variation or attenuation of the radiation beams along the plurality of interesecting paths is reconstructed into an image of the planar region of the patient. The thickness of the beams along an axis generally transverse to the planar slice affects the thickness of the planar slice examined. The width of the beams affects the resolution of the reconstructed image. The width is an axis within the plane which is generally transverse to the thickness axis and transverse to the path between the source and the detector. The radiation detectors generally consist of a scintillation crystal positioned to receive radiation and a photomultiplier tube optically coupled to the scintillation crystal. Alternately, the detectors may consist of a scintillation crystal optically coupled with a photodiode, a solid state radiation detector or an ionizable gas detector.
Generally, medical diagnosticians achieve the preferred results from tomographic scanners which have high resolution and low noise. A major factor in determining the resolution is the width of the radiation beams. The width may be determined by the width of the radiation receptive surface of the detector or by a source collimator. A scintillation crystal-photomultiplier tube detector generally has the scintillation crystal mounted in a support behind an aperture. The width of the aperture limits the width of the beam for reconstruction purposes. Additional radiation outside the beam which does not impinge on the receptive surface of the detector does not contribute to the reconstructed image even though it may pass through the patient.
Noise degrades the tomographic image. Generally, the amount of noise is related to the inverse square root of the number of photons of radiation received by the detector. Increasing the radiation receptive surface of the detector decreases the noise.
Accordingly, there is usually a trade-off between noise and resolution. Increasing the width of the radiation receptive surface of the detector reduces noise but also reduces resolution. Decreasing the width of the detector increases resolution but also increases noise.
The present invention contemplates a new and improved radiation detection apparatus which overcomes the above problems and others. The present invention contemplates a radiation detector which improves resolution without a corresponding increase in noise. Alternately, the present invention provides a detector that reduces noise without decreasing resolution.